Exocytosis

Exocytosis and Polarized Membrane

  • Exocytosis: The process where a vesicle adheres to the plasma cell and releases its contents.

  • Secretion in Animal Cells:

    • Animal cells secrete various proteins (e.g., digestive enzymes, transmitters).

    • These proteins are not released uniformly across the cell membrane.

  • Polarized Membrane:

    • Refers to when proteins are released from only one part of the cell.

    • Example: Digestive enzymes released from a specific region to avoid digestion throughout the cell.

Vesicle Movement and Membrane Domains

  • Vesicle Targeting:

    • Vesicles move to localized regions or subdomains within the outer membrane.

    • Signals on the outer membrane dictate what binds to the vesicle's lumen and gets released.

  • Lipid Layer Determination:

    • The layer in which lipids are located in the endoplasmic reticulum (ER) dictates where they will be found in the plasma membrane.

  • Vesicle Movement Pathway:

    • ER → Golgi → Plasma membrane

  • Microtubules and Vesicle Movement:

    • Vesicles move along microtubules to reach the plasma membrane.

    • Experiment using colchicine:

      • Colchicine, isolated from plants, disassembles microtubules.

      • Without microtubules, vesicles cannot move.

Regulated Secretion and Calcium's Role

  • Types of Secretion:

    • Constitutive Secretion

    • Regulated Secretion

  • Regulated Secretion Mechanism:

    • A ligand (e.g., neurotransmitter, hormone) binds to a receptor on the cell surface.

    • This binding causes the vesicle to fuse and release its contents.

  • Calcium's Importance:

    • Ligand-receptor binding leads to an increase in intracellular calcium levels.

    • Calcium is essential for vesicle fusion with the membrane.

  • Experimental Evidence:

    • Injecting pancreas cells with calcium alone induces exocytosis.

  • Calcium's Role in Exocytosis:

    • Extracellular ligands binding to receptors increases calcium levels inside the cell, which allows exocytosis to occur.

  • Further Discussion:

    • The mechanism by which calcium enters the cells will be discussed in later chapters.